1. Field of the Invention
The present invention relates to a novel steam turbine blade which is an axial entry, shroud-integrated type grouped blade, and a steam turbine using the steam turbine blade and a power plant with the steam turbine.
2. Description of the Related Art
A steam turbine blade has blade roots formed in various shapes. A turbine rotor is formed with blade grooves formed complementarily to the blade roots. The steam turbine blade is attached to the turbine rotor by fitting the blade roots to the blade grooves. One of the known structures of the blade root included in the steam turbine blade is a fir-tree axial entry blade root. In general, the fir-tree blade root is shaped symmetrically with respect to its centerline. A plurality of pairs of hooks are disposed on both sides of the centerline. During rotation of the turbine rotor, the plurality of pairs of hooks support a centrifugal force acting on the steam turbine blade.
It is a general method to join together a plurality of steam turbine blades through at least one of the platform and the shroud so as to form a grouped blade. Such a grouped blade has higher rigidity than that of a single steam turbine blade; therefore, they have an effect of suppressing vibration of the steam turbine blade during the operation of a turbine.
FIG. 9 is a perspective view illustrating the grouped blade structure of the conventional steam turbine blade. One of methods of forming a grouped blade is as below. A shroud attached to the tip of a blade is made of a member different from the blade. Portions called tenon disposed at the blade tips of a plurality of steam turbine blades are riveted. Thus, the blades of the plurality of steam turbine blades are joined to the single shroud to form a grouped blade. However, such a method has a probability that variations of tenon riveting work may vary strengths of riveted portions.
There are grouped blade structures that allow for the improvement of such a problem of the conventional method. In one of the grouped blade structures, a plurality of steam turbine blades have a common shroud and a common platform, and blades, the shroud, blade roots and the platform are integrally molded. For instance, JP-A 53-126409 describes a grouped blade structure in which a blade root is of fir-tree axial entry and three blades and two blade roots are integrally molded.
JP-A 1-300001 describes a grouped blade structure in which a blade root is of axial entry and has a T-head provided with only one pair of hooks relative to its centerline, and three blades and three blade roots are integrally molded.
JP-A 10-184305 describes a grouped blade structure in which two adjacent blades are integrally molded by one shroud, one platform and one blade root.